Unions for the connection of tubing typically fall into one of two categories, those for high-pressure and those for low-pressure. High-pressure unions typically use either one or more tapered ferrules that fit around the outside diameter (OD) of the tubing. The bore of the union is shaped to accept both the OD of the tubing and a mating taper to that of the ferrule. A nut, which has a thread mating with that on the body of the union, which is in contact with the ferrule or ferrules, is tightened to provide compression of the ferrule into the body of the union. In this manner, the ferrule forms a liquid- and gas-tight seal between the outer surface of the tubing and the inner surface of the union. This type of union and mating action is typically referred to as frusto-conical compression. There are numerous examples of this frusto-conical compression in the prior art. Fields (U.S. Pat. No. 3,880,452), for example, with improvements by Worden (U.S. Pat. Nos. 4,991,883; 5,163,722; 5,234,235). Henion, Sheppard, Wachs (U.S. Pat. No. 5,587,582) utilized the frusto-conical approach substituting a “tee” junction for the union so that three pieces of tubing could be connected. Fatula (U.S. Pat. No. 4,529,230) simplified and improved this approach by reversing the direction of the taper and combined the two ferrules into a single unit, thus reducing the number of sealing surfaces from three to two. Jones, Porter, Kelly (U.S. Pat. No. 6,193,286) utilized spring loaded tension to push the tubing into the union coupling body in an attempt to ensure that the tubing is fully seated for frusto-conical compression. Davis, Stahl, Hefta, and Lee (Anal. Chem., Vol 67, p 4549-4556, 1995) and Bateman, White, and Thibault (Rapid Comm. Mass Spectrom., Vol 11, p. 307-315, 1997) summarize the use of frusto-conical high pressure couplings with capillary LC.
Low-pressure connections with small diameter capillary tubing are commonly made using a short segment of elastomeric tubing such as Teflon®, Teflon®-PFA, or KalRez® perfluoroelastomer as a coupling sleeve or union. The inside diameter (ID) of the elastomer tubing is chosen to be slightly smaller than the OD of the capillary tubing being connected. The two pieces of capillary tubing to be joined are press fit into respective ends of the elastomeric sleeve until their ends make direct contact for a butt connection. The interference fit between the ID of the sleeve and OD of the tubing results in strong friction between the coupling sleeve and the capillary tubing; holding the tubing in place. Various examples of this type of simple one-piece union appear in the literature: e.g. Gucek, Vreeken, Verheij (Rapid. Commun. Mass Spectrom., Vol 13, p. 612-619, 1999); Guzman (LC-GC, Vol. 17, 1999); Alexander, Schultz, Poli (Rapid Commun. Mass Spectrom., Vol 12, p. 1187-1191, 1998); and Herring, Qin (Rapid Commun. Mass Spectrom., Vol 13, p. 1-7, 1999).
Lowe (U.S. Pat. No. 822,530) teaches an embodiment in which an elastomer tube within a coupling body is used to connect two pieces of tubing. A combination of axial and radial compression, provided by tapered sections within the coupling body, is applied to the elastomer forms a seal between the two pieces of tubing,
Each method has disadvantages. The high-pressure fittings can be difficult to use and often have many parts to assemble. It is typically difficult to verify the integrity of the coupling, which often requires partial disassembly. To overcome this particular limitation, Worden's patents ('883, '722, '235) use a removable, central seating element and spring loaded ferrules designed so the proximal ends of connecting tubing make direct contact with the central seating element rather than each other. This design overcomes the assembly problem of a conventional frusto-conical union, however, a significant amount of swept volume is added to the connection. The two pieces of tubing fit into the seating element and are separated by the length of the seating element.
A significant limitation of the apparatus disclosed by Lowe (U.S. Pat. No. 822,530) is a mechanical design that is overly constrained in the axis of the connected tubes. The co-axial alignment of the tubes being joined is highly dependant on the coaxial alignment of bores in the coupling body to both the bore of the elastomer tube and the outside surface of the tubes being joined.
The low-pressure sleeve approach can be quite efficient and provides an easy to use one-piece, see-through design. Use at high pressures (e.g. greater than 100 to 150 psi) is not routinely possible since the sleeve expands and distorts under pressure and the assembly does not hold together. Therefore, there is a need for an improved methodology and hardware for minimal volume capillary connections.